Microwave spectrum analyzers and related frequency-scanned, superheterodyne microwave receivers typically employ a tunable yttrium-iron-garnet (YIG) bandpass filter, or YIG tuned filter (YTF), as a preselector filter. The YTF preselector filter helps to prevent or at least minimize the presence and effects of various unwanted mixing products in an output of a first frequency conversion stage of the spectrum analyzer. In general, to be effective in as a preselector, tuning of the YIG tuned filter must be synchronized to a frequency tuning or sweeping of the spectrum analyzer. In particular, a center frequency of the YIG tuned filter must track or correspond to a scan frequency of the spectrum analyzer. In addition, at an end of a frequency sweep, tuning of the YIG tuned filter must be returned to a start frequency before a next sweep can be started.
In most cases, the YIG tuned filter is tuned by applying a tuning signal, typically a linear ramp voltage or ramp current, to a tuning port or input of the filter. The linear tuning signal is often generated using a digital-to-analog converter (DAC) and is based on a known start frequency, stop frequency, and sweep time of the frequency sweep being performed. In turn, a sweep controller of the spectrum analyzer simultaneously controls a frequency of a tunable local oscillator of the first frequency conversion stage and the DAC output used to tune the YIG tuned filter. Thus, the YIG tuned filter by virtue of the application of DAC-generated tuning signal, effectively tracks a frequency sweep of the spectrum analyzer.
Unfortunately, YIG tuned filters generally have tuning responses that generally are not particularly linear. The observed non-linearity of the tuning response of a typical YIG tuned filter results in an effective tracking error between an intended and an actual tuned frequency of the YIG tuned filter. The tracking errors associated with non-linearity of the YIG tuning response adversely affect an accuracy of amplitude measurements made with the spectrum analyzer, among other things.
In addition, YIG tuned filters typically exhibit a hysteresis in the tuning response. The tuning hysteresis is manifested by a non-linear time lag between the tuning signal and the tuning response. The presence of such a tuning hysteresis generally necessitates the application of a ‘de-hysteresis’ pulse in the tuning signal of the YIG tuned filter during a retrace or retuning of the YIG tuned filter. The de-hysteresis pulse is applied when transitioning from a stop frequency back to a start frequency at an end of a frequency sweep. In most cases, the de-hysteresis pulse momentarily tunes the YIG tuned filter below the start frequency so that the filter tuning always approaches the start frequency from ‘below’ at the beginning of each sweep. By approaching the start frequency from below and in a consistent manner at the beginning of each sweep, the effect of tuning hysteresis is minimized. Unfortunately, the de-hysteresis pulse and a settling time associated therewith increase a time between successive frequency sweeps. As a result, an overall sweep repetition rate of the spectrum analyzer is effectively reduced from using the de-hysteresis pulse in YIG tuned filter tuning.
Accordingly, it would be advantageous to have an approach to tuning a tunable device, such as YIG tuned filter, that reduced, or preferably minimized, the effects of tuning non-linearity and tuning hysteresis. Such a tuning approach would solve a long-standing need in the area of tunable devices, such as YIG tuned filters, used in swept-frequency microwave spectrum analyzers and related scanning superheterodyne microwave receivers.